The present invention relates to an apparatus and method for locating a fault within an electric circuit, and more particularly to a fault locating method and apparatus in which a device for sensing current is synchronized with a signal generating source for producing a current to be detected, which current provides an indication of the location of the fault.
In the field of automatic in-circuit electrical component testing, it is sometimes necessary to utilize a manually positioned current probe to identify a faulty component, such as a gate, with particularity. Typically, in the operation of an automatic in-circuit electronic component tester, such as those illustrated, for example, in U.S. Pat. Nos. 3,870,953 and 4,216,539, the tester is connected to various nodes or busses on a printed circuit board containing the component to be tested. Stimulus signals are injected at some of these nodes and the response of the component to these signals are detected at other nodes and processed to determine whether the component is functioning properly. Since a number of different components can be connected to a single node, the tester is not always capable of identifying the particular component that causes a faulty signal to be produced on such a node. In this case, it becomes necessary for the operator of the tester to utilize a hand-held current probe to detect the value of the current flowing into or from each of the various components connected to the node at which the improper response signal appears.
Use of the current probe typically involves injecting a signal onto the node, or common bus, producing the improper response signal. For example, in the testing of digital equipment, if the signal at a faulty bus improperly remains in one binary state because a gate connected to the bus is faulty, pulses of an opposite polarity from that binary state are applied to the bus, to thereby produce a current flow. The operator of the test equipment places a current probe on the lead connecting each of the possibly faulty gates to the bus. The lead connecting the faulty gate to the bus will carry the greatest current, to thereby indicate the particular faulty component. A similar approach using a complementary signal can be used during the testing of analog equipment.
One prior art method for processing the output signal produced by a current probe has been to square the output signal so that all pulses in it are of the same polarity and then determine a DC value for the signal based upon the amplitudes of the squared pulses. The component producing the highest DC value is identified as the faulty component.
One problem associated with this type of signal processing is the fact that it is dependent upon the duty cycle of the pulses applied to the bus. More specifically, the DC value is determined by integrating the value of each pulse over time. Therefore, in order to obtain the highest possible DC value for each output signal, and thereby obtain the greatest resolution, the duty cycle of the applied pulses should be high to reduce the time between pulses during which the DC signal decays. However, the pulses that are applied to the bus operate to back drive each component connected to the bus, thereby causing the temperature of the components to rise. If the duty cycle of the applied pulses is too high, the temperature threshold of the components will be exceeded, causing them to break down. Thus, it is necessary to compromise the resolution that can be obtained from the current probe output signal in view of the thermal capabilities of the electrical components.
Another problem associated with prior art current probes used for in-circuit electronic component testing is the fact that the probes are responsive to stray currents, electromagnetic noise and the like, as well as the applied stimulus pulses. For example, a circuit board might contain a number of conductors running parallel to one another. Since most known current probes operate on either inductive priciples or the Hall effect, a current probe used to detect the current flowing in one of the conductors will be affected by the electromagnetic fields generated by currents flowing in any of the other nearby conductors.
Accordingly, it is a general object of the present invention to provide a novel current sensing device that is capable of providing high signal resolution and indicating relative current direction in the presence of noise.
It is another object of the present invention to provide a novel current sensing apparatus that is not dependent upon the duty cycle of an injected stimulus signal to obtain good resolution in an output signal.
It is a further object of the present invention to provide a novel current sensor that is synchronized with an applied stimulus signal to thereby reduce the response of the sensor to undesirable noise in the system being tested.
It is yet another object of the present invention to provide a novel method for processing the output signal of a current probe to reduce the effects of noise on the signal that is desired to be measured.
It is yet an additional object of the present invention to provide a novel current probe signal processing circuit in which the signal is sampled and integrated to further eliminate the effects of unwanted noise on the current measurement process.
It is a particular object of the present invention to provide a novel current probe circuit for use with an automatic in-circuit component tester.
One system for synchronizing a current sensor output signal to an applied stimulus signal is disclosed in U.S. Pat. No. 4,074,188. In that system, a synchronizing signal is applied as an enabling signal to a comparator that compares the sensor output signal to one or more threshold values. The present invention offers a different approach, utilizing a sample and hold technique, for synchronizing the applied and measured signals. The particular manner in which the present invention synchronizes the signals, and thereby achieves the foregoing objects and advantages, will become apparent to a person of ordinary skill in the art upon a persual of the following detailed description of a preferred embodiment thereof, when taken in conjunction with the accompanying drawings.